Fix concurrent access to repository

Build fails when installing runtime while a runtime or an app is pushed
to the repository. We can't use rsync to upload a runtime or an app.

This download commit metadata from the repository before build, then upload
using `ostree-push` instead of `rsync`. The repository is then updated
using `flatpak build-update-repo`.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>